Hydraulic valves

ABSTRACT

A pilot actuated hydraulic control valve has a flow setting member in the form of a valve piston an end of which co-operates with radial outlet ports of the valve to form a variable metering orifice. A flow sensor provides a feedback differential pressure proportional to the flow through the valve and that feedback differential is used to control a pilot stage which actuates the flow setting member.

This invention relates to valves, and in particular to hydraulic controlvalves.

A pilot actuated hydraulic control valve is known from U.K. Pat. No.1335042, in which the lands and the intermediate recesses of acylindrical valve spool housed slidably in a matching valve bore,co-operate with inlet and outlet ports radially entering the valve boreto provide several variable metering orifices whose apertures aredependent on the positions of the lands and recesses relative to theports. The axial location of the spool, which determines these relativepositions, is varied by applying different actuating pressures to thetwo ends of the spool. The pilot valve through which the actuatingpressures are applied to the two spool ends, is of similar construction,with the axial location of its spool being determined by both a settingforce applied to it by a force motor, and a feedback pressuredifferential derived at a flow sensor placed in a main flow path.

Because the effective surface areas of the facing surfaces of the twolands adjoining each recess are equal and the pressure acting on themare the same, the spool of the aforementioned control valve is nominallybalanced with respect to the pressures in each of its main flow paths.Consequently, the actuating pressures which need to be applied to thespoolends are largely unaffected by the pressures in the main flowpaths.

The aforementioned prior art valve performs satisfactorily over a widerange of applications but has an undesirably complicated structure forapplications in which directional control of the fluid flow is notrequired.

The present invention provides a flow feedback responsive pilot-actuatedhydraulic control valve in which fluid flow is metered by varying theaperture of a metering orifice through axial movement within a valvecavity, of a fluid pressure operated valve piston an end surface ofwhich is subject to the pressure of the fluid at the inlet to themetering orifice.

Conveniently, one or more radial outlet ports from the cavity cooperatewith said end of the piston to form the metering orifice, and the inletin coaxial with the valve piston.

The pilot stage will usually comprise a spool valve operated byelectromagnetic flow setting means such as a proportional solenoidacting on the pilot spool against a restoring force provided by a biasspring, and the pilot stage will respond to electrically orhydraulically transmitted feedback from a flow-sensing device having avariable orifice, a fixed orifice, or both.

In one form of the present invention, the hydraulic control valve has amain stage the valve piston of which is a stepped valve piston slidablyhoused in a matching stepped cavity, said end being the end of the valvepiston which has the smaller effective end area, and a pilot stage whichcontrols the proportion of the pressure at the inlet to the meteringorifice which is applied to the larger effective end area of the steppedvalve piston.

In an alternative form the main stage of the hydraulic control valve hasa collared valve piston having equal effective areas at both ends andone or more end-to-end fluid ducts, the collar providing oppositelydirected actuating surfaces of equal effective areas to which theactuating pressures, provided by pilot valve, are applied to meter theflow. The fluid duct or ducts ensure that both ends of the valve pistonare exposed to the pressure at the inlet to the metering orifice.

The valve may be constructed as in-line flow control valve in which themetering orifice and the flow-sensing device are in series, or the valvemay alternatively be constructed as by-pass valve, with the flow-sensingdevice and the metering orifice lying in parallel. In this second formthe valve may be used to control the flow or the pressure applied to aload.

Since the valve piston can be made shorter than an equivalent spool, thepresent invention provides a unidirectional control valve of fairlycompact and simple construction. Moreover, on account of a simple flowpattern its parasitic flow resistance, that is flow resistance otherthat that due to the metering and flow sensor orifices, will generallybe less than that of a prior art spool valve of similar externaldimensions.

The present invention shall now be described further by way of exampleonly and with reference to the accompanying drawings of which:

FIG. 1 is a schematic section of an in-line flow control valve accordingto the invention;

FIG. 2 is a schematic section of a modified form of the valve of FIG. 1;

FIG. 3 is a schematic section of a by-pass valve according to theinvention;

FIG. 4 is a schematic section of a modified form of the valve of FIG. 3;

FIG. 5 is a schematic section of another modified form of the valve ofFIG. 3;

FIG. 6 shows details of modifications to the valves shown in FIGS. 1 and3;

FIG. 7 shows details of further modifications of the valves of FIGS. 2,4 and 5; and

FIG. 8 is a schematic section of an alternative form of a flow controlvalve according to the invention.

FIG. 9 is a schematic section of a flow control valve such as shown inFIG. 8 modified by a pressure transducer of the valve of FIG. 2.

Referring first to FIG. 1, a cartridge-type in-line flow control valve 1includes a main stage located within a main valve block 2, a pilot stagehoused within a pilot valve block 3, and a proportional solenoid 5. Themain valve block 2 and the pilot valve block 3, which is secured to themain valve block 2 by several bolts 15 of which two are shown, togetherconstitute the valve body 4 which itself is mounted on a valve base 6and fixed thereto by bolts such as shown at 7.

The main stage comprises a stepped valve piston 9 whose large diameterportion 23 (diameter D) and small diameter portion 25 (diameter d) makea sliding fit with the large diameter section 12" and the small diametersection 12' respectively of the stepped cylindrical cavity 12 formed bya centrally located through-bore in the main valve block 2. Acompression spring 57 is interposed between the large diameter endfaceof the piston 9 and the facing part of the pilot valve block 3. Atubular extension, tube 16 of the main valve block 2, and theflow-sensor housing 24 which is integral with the extension 16accommodated within a bore 17 in the housing 6, has an axial valve inletport 18 leading to a valve chamber 58, and four radial valve outletports 11. A sharp-edged variable metering orifice is provided by theinteraction of the annular groove 60 in the valve block 2 and theV-shaped recesses in the small diameter end 61 of the piston 9. Theoutlet ports 11 and the valve chamber 58 via the inlet port 18,communicate respectively with fluid passages 13 and 19 in housing 6.O-ring seals 21 and 22 provide for leak-tight contact with the wall ofthe bore 17.

The flow sensor housing 24 contains a flow sensor 10, comprising anaxially moving bobbin 27 guided within a central hole 28 of a spokedsupport ring 29. The bobbin is spring-loaded, by means of a coil spring30 such that its mushroom head 62 seats against the throat 31 in theabsence of fluid flow. The optimum angle between the throat 31 and thebevelled surface of the mushroom head is approximately 35°.

The pilot stage assembly, which is housed in the pilot valve block 3,consists of a pilot spool 35 enclosed in a pilot valve bore 36. Thepilot spool is guided axially within sealing bushes 37 and 38 at itsouter ends, with biasing springs 39 and 40 being provided to return thespool 35 to its nominal null-position, as well as to overcome stickingdue to friction, and provide sufficient stiffness for good dynamicperformance. The null position of the pilot spool 35 is set byadjustment of the bias of spring 40 through rotation of the threadedstud of the null adjuster 41 thereby to lower or raise the attachedplatform on which spring 40 rests. A connecting rod 42 connects thepilot spool to the armature 43 of the solenoid 5. The pilot spool 35carries end lands 44 and 46 separating end chambers 47 and 50 of thepilot cylinder 36 from the pilot chambers 48 and 49 respectively. Thechambers 48 and 49 are separated by a double land 45 located midwaybetween end lands 44 and 46. The double land 45 is dimensioned so as tobe in underlap with the radial, drilled port 56 when centred withrespect to port 56.

The valve body 4 incorporates internal fluid ducts 51, 52, 53, 54, withO-ring seals 55 preventing leakage at the boundary of main valve block 2and pilot valve block 3, and, in the case of fluid duct 54, also betweenmain valve block 2 and housing 6. The valve body 4 also containsdrainage ducts, designated R, S, T, and U in the drawing which areconnected to a fluid tank at atmospheric pressure. In practice, not allthese ducts would normally be in the same plane, but, in order to aidthe understanding of the invention, are shown in the drawings as lyingin the plane of the section. All the internal ducts are formed bydrilling from the outside and subsequent plugging as shown at 887, FIG.8. Drainage ducts S and U serve to drain off any hydraulic fluid seepingpast sealing bushes 37 and 38, and drainage duct R prevents the build-upof pressure in the annular area behind the large diameter portion of thepiston 9 and thus serves to decouple the fluid pressures at oppositeends of the piston. Drainage duct T is the drain for fluid suppliedunder pressure from valve chamber 58 via fluid passages 32, 33, radialpassage 34 and circumferential groove 34' of the piston 9 and fluid duct52 to pilot chamber 49. The pressure differential developed in operationof the valve across the flow sensor 10 is transmitted to end chambers 47and 50 by way of ducts 51 and 54 respectively. Fluid pressure in theport 56 of the pilot valve is applied to the large diameter endface ofthe piston 9 through duct 53.

In operation of the valve 1, supply pressure P_(o) is present at inletport 18, the load such as a hydraulic actuator (not shown) beingconnected to the fluid passage 13. When the valve is in the closedposition, as shown in FIG. 1, the piston 9 rests against the shoulder ofthe stepped bore 12 and the outlet ports 11 are blocked off by the smalldiameter portion 25 of the piston 9, the pressure P₁ within the valvechamber 58 being equal to the supply pressure P_(o). When the doubleland 45 is centred with respect to the port 56, then, on account ofequal pressure drops at the two lands of double land 45, the pressureP_(c) applied to the large diameter endface of the piston 9 is equal toone half the pressure P₁. The diameters D and d are chosen such thattheir effective areas are in a ratio of 2:1 and therefore the net forceon the piston 9 due to pressures 1/2P₁ and P₁ is zero, and the piston 9is stationary. A necessary bias for the piston 9 towards the closedposition is obtained by adjusting the null position of the double land45 of the pilot spool 35 to be off centre with respect to the port 56such that the pressure P_(c) is somewhat larger than half the pressureP₁, a further small bias being provided by the spring 57 acting on thevalve piston 9. Furthermore, when the valve is closed, the feedbackpressure differential across the pilot spool 35 between end-chambers 47and 50 is zero.

In order for flow to commence, the proportional solenoid 5 is energisedwith a current proportional to the required flow. As a result the pilotspool 35 is moved downwards by some distance against the bias spring 40.The consequent decrease in the gap between the pilot chamber 49 and theport 56, coupled with an increase in the gap leading from the port 56 tothe pilot chamber 48, which by virtue of its connection to the drainageduct T is at atmospheric pressure, leads to a reduction in the pressureP_(c) in the port 56 and hence on the large endface of piston 9. Thepiston 9 therefore lifts off the shoulder in the stepped bore under thegreater force now acting on the small endface, and fluid begins to flowthrough outlet ports 11 to the load.

As soon as fluid begins to flow in the main flow path leading from fluidpassage 19 to the outlet ports 11, in sufficient quantity for theflow-sensor bobbin 27 to rise off the throat 31, a pressure differentialis developed across the flow-sensor 10 which is applied in theabove-described manner across the pilot spool 35, bringing the feedbackarrangement into operation, that is to say, the pilot spool is nowsubject also to a hydraulically transmitted feedback force provided bythe pressure differential between the ends of the spool 35. Depending onwhether the flow is less or greater than the required flow, this forcewill be less or greater than that applied by the solenoid 5.Accordingly, the pressure P_(c) acting on the large endface of piston 9will be nominally less or greater than 1/2P₁, after allowing forhydrodynamic forces acting on the piston, and the piston will move toincrease or decrease flow through the valve. Since the pressure in endchamber 47 is lower than (or at most equal to) that in end-chamber 50,which is at supply pressure P_(o), the force on pilot spool 35 due tothe pressure differential always opposes that provided by the solenoid,the force being the greater the larger the flow through the flow-sensor10. When the flow through the valve is at the desired rate, the forceson the pilot spool 35 balance such that the double land 45 is nominallycentred with respect to the annular port 56, i.e. the pressure P_(c)=1/2P₁, and the piston is stationary.

Valve 201 of FIG. 2 is another embodiment of an in-line control valve ofthe present invention. The value 201 is essentially the same as valve 1in respect of the combination and inter-action of the integers making upthe valve, but shows various modifications in the detailed constructionto which the following description will by-and-large be confined.

The main differences between the valve 1 and the valve 201 are thelocation of the flow sensor 210, itself a variant of the flow sensor 10,in the valve outlet rather than in the valve inlet, the use of apressure transducer 291, which converts the feed-back pressuredifferential into an equivalent electric feed-back signal, and theadaption of the valve base 206 to accommodate the altered flow sensorlocation, and to permit gasket mounting of the valve 201. Further minorchanges involve the simplification of the valve piston 209 and the pilotvalve 214, which follows from the elimination of some of the internalfluid ducts made possible by the conversion of the feed-back pressuredifferential into an electric feed-back signal.

Considering some of the aforementioned differences in more detail, amodified valve base 206 has inlet and outlet passages 219 and 213' bothof which terminate in the planar mounting inter-face 292 of the valvebase 206. In use, the valve 201 is mounted in a conventional manner on amatching inter-face of some other hydraulic component (not shown) with agasket being interposed between the adjacent faces. The inlet passage219 leads into a co-axial stepped bore 217' into which is inserted thetubular casing 216 housing the valve piston 209. The tubular casing 216roughly corresponds to the tube 16 of valve 1, and is provided with theradial outlet ports 211, which together with the lower end 261 of thevalve piston 209, form the metering orifice. A slightly increasedoverlap between the end of the valve piston 209 and the radial outletports 211, reduces fluid leakage when the valve 201 is shut off. Fluidpassage 213 connects the stepped bore 217' to the stepped bore 217"which contains the flow sensor 210. The valve casing 216 is clamped intoposition by a clamping block 202, which is secured to the valve base 206and which also incorporates fluid ducts 254, 256 S and T.

As indicated above, the flow sensor 210 is a variant of the flow sensor10. The flow sensor 210 is in the form of a cartridge inserted into theboard 217", and comprises a tubular flow sensor housing 224 secured toan end plate 293 of the cartridge.

Guided within the housing is the straight-sided barrel-shaped poppet227, taking the place of the bobbin 27, whose open end faces the endplate 293. The poppet 227 is spring loaded by the coil spring 30, whosepre-load is adjustable with the aid of a setting screw arrangement 294situated in the end plate 293, and the variable orifice of the flowsensor 210 is, as before, an annular orifice bounded by the bevellededge of the poppet 227 and the throat 231 in the flow sensor housing224. A small diameter fixed orifice 295 is provided in the poppet 227 toprovide a measurable feed-back pressure differential even at very lowflow rates, thereby reducing the minimum flow which can be metered bythe valve. Fluid entering the interior of the poppet 227 at these lowflow rates is discharged through an opening 295 in the side wall of thepoppet and hence to the outlet passage 213' through openings in the flowsensor housing 224. The point at which the flow sensing action changesover from the fixed orifice 295 to the annular orifice, i.e. the pointat which the poppet 295 lifts off the throat 231, is determined by thepreloading of the spring 30. At higher flow rates the contribution ofthe fixed orifice to the total feed-back pressure differential becomesnegligibly small due to its very much higher flow resistance, while atthe same time the opening 295' maintains the area of the poppet 227 atthe secondary, i.e. the oulet pressure of the flow sensor.

Irrespective of whether the pressure differential is developed acrossthe fixed or the variable flow sensor orifice, it is applied via fluidducts 254 and 251 to the cantilever beam of a conventional cantileverbeam pressure transducer assembly 291 mounted on the valve base 206. Thepressure differential many, for instance, be applied to the beam bymeans of two diameter-matched pins. Alternatively the pressuredifferential may be applied to a single pin and directly to the beam.The drawing illustrates a two-pin version in which the cantilever beam,which is shown lead-on, bends under the action of the pressure appliedto the outer ends of the two pins and converts the applied pressuredifferential into an equivalent electric feed-back signal to be used tocontrol the solenoid 5. This could be done by comparing a demand signalwith the feed-back signal and applying the resultant error signal tocontrol the solenoid.

The pilot valve 214, which is positioned with its axis at right anglesto the axis of the main valve piston 209, comprises a landed spool 235supplied via a washed filtered 269 and fluid duct 254 with the valveinlet pressure P_(O) ' and controlling the proportion of the pressurewhich is applied as control pressure P_(C) to the larger end face of thevalve piston 209. Bias springs 39 and 40 and the zero adjustmentmechanism 41 are provided as the earlier described valve 1. Employing anelectric signal to control the solenoid 5 enables a simplification ofthe pilot valve spool 235 to a two-land spool, and thus contributestowards reducing friction in the pilot valve 214 compared to that in thepilot valve 14 of FIG. 1.

The by-pass valve shown in FIG. 3 is constructed in a manner similar tothe valve of FIG. 1 and identical components are referenced by the samenumeral, the major constructional difference between the two valvesbeing the provision of two outlet passages in the housing 306, of whichpassage 397 leads to the load (not shown), and passage 313 forms abypass line for the load and leads to a tank at atmospheric pressure. Asin the previously described valve, the pressure differential across theflow-sensor which is now housed in the outlet passage 399, istransmitted to the end chambers 47 and 50 of the pilot valve cylinder36. Fluid flow to the load is now controlled by regulating the amount ofthe flow discharged via the by-pass line, and as a consequence thedirection of flow through the pilot stage, past the double land 45, mustbe reversed, that is, fluid passage 352 now connects the valve chamber358 to pilot chamber 48, and pilot chamber 49 is connected to tank. Itwill be readily seen that, with the other forces on the pilot spool 35being the same as before, the piston 9 will now open the valve when thefluid flow to the load is to be reduced, and close when it is to beincreased.

Another major difference between the valves 1 (and 101) and 301 (and 401and 501) is found in their operational characteristics if the pressuresextant in their respective inlet passages, 19 and 319 and outletpassages to the load, 13 and 397, are compared. In both cases the outletpressure P₂ depends, of course, on the load resistance, that is to say,the pressure P₂ is low if the load resistance is low, and the pressureP₂ is high if the load resistance is high. However, while in valve 1 thesupply pressure P_(o) is substantially unaffected by variations in theload resistance since the flow-controlling metering orifice lies betweenthe supply pressure inlet and the outlet passage to the load, thepressure P_(o) in valve 301 is always equal to the pressure P₂, save forthe comparatively small difference due to the pressure drop at theflow-sensor 10, because fluid flow to the load is controlled byregulating the rate of discharge to the tank through the outlet passage313. This difference has important consequences in that it will ingeneral be necessary to employ an accurately settable pressure reliefvalve (not shown) in the pressure supply line to valve 1, through whichfluid is discharged almost continuously in order to prevent pressurebuild-up on its inlet side even under normal opening conditions; whilefor the by-pass valve 301 only the addition of a more simpleover-pressure relief valve is required to provide a safety valve in caseof malfunction. Also, the by-pass valve 301 will generally be moreenergy efficient, since fluid is discharged to the tank at a pressurecommensurate with the load resistance, rather than at full supplypressure as in the case of the valve 1 where the pump (not shown) whichsupplies the hydraulic fluid to the valve has to operate at full loadeven when the flow demand is zero and all the fluid passes through theseparate pressure relief valve. Notwithstanding the generally greaterenergy efficiency of the by-pass valve 301, the valve 1 will, forinstance, be used where fluid flow to a load with fairly constant loadresistance is to be controlled, or where simultaneous control isrequired of two or more individually controlled load circuits suppliedfrom a common pressure supply.

A pressure responsive over-ride may be used with the by-pass valve 301,which may be either of electrical or mechanical (including hydraulic)nature, acting respectively via the current supply to the solenoid 5 ordirectly on the pilot stage 14. With the aid of such an over-ride andunder conditions of very low or zero fluid flow to the load, that iswhen the pressure differential across the flow sensor 10 is nearly zeroso that practically no feedback pressure is acting on the pilot spool35, the by-pass valve 301 may operate to control the pressure applied tothe load.

FIG. 4 shows a by-pass valve 401 incorporating substantially the samemodifications with respect to the valve 301, FIG. 3, as the valve 201 ofFIG. 2, with respect to valve 1 of FIG. 1, save that the inlet andbypass outlet passages 419 and 413, and the metered outlet flow passage497 respectively terminate in two planar mounting faces at right anglesto each other. Also, the pilot spool 435 and the pressure supply duct454 to the pilot valve are reversed as compared to the spool 235 andpressure supply duct 254 of FIG. 2, in order to obtain a pilot-valvefail safe operation, which causes the by-pass to be fully opened in thefailure mode.

FIG. 5 illustrates an alternative design of the valve 401, of FIG. 4, inwhich the valve and the flow sensor are symmetrically disposed about thevalve inlet passage 519. This allows the provision of a single mountinginterface for gasket mounting or has shown in FIG. 5, face mountingusing O-ring face seals such as at 598. The only other noteworthy changelies in the necessity to provide a separate end plate 599 to clamp thetubular casing 216 of the main valve into position.

FIG. 6 illustrates some possible modifications of the throttling piston9 of valves 1 and 301, the outlet ports 11 thereof and the flow sensor10. Instead of the sharp-edged variable orifice being provided betweenthe sides (and apex) of the inverted V's 61 of the piston 9, and theannular groove 60 as is shown in FIGS. 1 and 3, the circular outletports 611 now terminate flush with the inside wall of the main valveblock, and the piston 609 has a flat end face, whose outer edge togetherwith the circular inside edge of the outlet ports 611 form the variablemetering orifice.

The modified flow sensor 610 has its spring assembly positioneddownstream of the flow sensor bobbin head 662. The modified flow sensor610 comprises a central shaft 671 on which the bobbin 627, which isjoined to a sleeve 672, is slidably mounted, the shaft being supportedby a tube 673 which extends outwardly from a support member 629. Thesupport member 629 is clamped in between the shoulder 674 of the housingand the lower endface of the tubular portion of the main valve block 3.A spring 630 surrounding the shaft 671 is placed in between the bobbinhead and a retaining ring 674 secured to that portion of the shaft 671which extends into the passage 32 of the piston 609. The advantages ofthis arrangement are reduced obstruction of the main flow path as wellas greater protection of the flow-sensor against damage during e.g.storage of the valve body 4 separate from the housing 6 (306).

FIG. 7 illustrates another way of converting into an electric feed-backsignal the feed-back pressure differential developed across the flowsensor 710, which is otherwise similar in construction to the flowsensor 201 of FIG. 2. In the flow sensor 710 the linear displacement,from its initial position, of the flow sensor bobbin 727 is measured bymeans of a linear variable displacement transducer, such as Sangama typeNA2. The amount of linear displacement is determined by the balance ofthe pressure differential developed across the flow sensor and theopposing returning force of the spring 30, and thus provides a measureof flow through the flow sensor. An electrical null-adjuster 740, whichhas to be set prior to insertion of the flow sensor cartridge into thevalve base, is provided in the flat end face of the flow sensor poppet727. Adjustment of the spring pre-loading is accomplished by selecting asuitable number of washers 791, interposed between the end of the springand the end plate 793. The opening 295 in the side wall of the poppet727 transmits the flow sensor outlet pressure to the interior of thepoppet. Using a linear displacement transducer eliminates the need for apressure transducer, such as 291 of FIG. 2, and the associated fluidducts, but does not give the same low flow range response as thealternative conastruction, since the signal is a function only of poppetdisplacement.

Referring now to FIG. 8, there is shown an alternative form of theinvention. A cartridge valve 801 designed to be fitted to a valve base6, comprises a control or main valve and a pilot valve. The main valvewhich regulates fluid flow from an inlet passage 19 to an outlet passage13 in the valve base 6, is controlled by the pilot valve, and the pilotvalve by a proportional solenoid 5 mounted on the valve body 804.

The valve body 804 comprises a main valve housing 802 and a pilot valvehousing 803. The main valve housing 802 has a valve bore 812 (i.e. 812'and 812") which extends form a block line portion, block 818, restingatop the valve base 2, into and through a tubular extension, tube 816,of the block 818, which (tube 816) is accommodated in the bore 810 ofthe valve base, to terminate in the inlet port 836 of the main valve803. The valve bore 812 is a stepped bore with its smaller diametersection 812' lying wholly within the tube 816 and terminating in theinlet port. The bore 812 widens into the larger diameter section 812"near the upper end of the tube 812 and extends through the remainder ofthe housing 802. Four radial outlet ports 811 are provided in the wallof the tube 816 near the inlet port 858.

The valve piston is in the form of a hollow cylindrical, collared flowcontrol member or sleeve 809 housed within the valve bore 812, itshollow interior constituting a single pressure equalising duct 813. Itslower end 826 makes a close sliding fit with the narrower section 812'of the valve bore, the annular lower end face 861 providing a meteringedge which co-operates with the outlet ports 811 to form the variablemetering orifice. The upper end 825 of the sleeve 809 is of the samediameter as the lower end 826 so that the sleeve 809 is pressurebalanced with respect to the inlet pressure P₁, and makes a closesliding fit with a bush 881 retained at the upper end of the valve bore812. The central portion of the sleeve 809 is shaped into a shoulderedcollar 827 providing the upper and lower actuating surfaces 828 and829'. The lower actuating surface 829 area is made up of the radialtransition surface 829' between the widest part, which makes a closesliding fit with the bore 812, and a region of intermediate diameter ofthe collar 827 spaced from the wall of the valve bore 812"; and afurther radial transition surface 829" between the intermediate diameterregion, and the lower end portion 826 of the sleeve 809 to which thefurther surface 829" also acts as lower end stop. A snap ring 888resting in a circumferential groove adjoining the shoulder 887 of thebore 812 between its narrower and wider sections 812' and 812", resultsin a minimal area reduction on account of the line contact made when thecollar 827 abuts it, and so helps to prevent "sticking" of the sleeve809 in that position. "Sticking" in the uppermost position of the sleeve809 is prevented by the provision of an oil relief slot 865 allowing oilto reach the upper endface of the sleeve 809.

A flow sensor 810 similar in construction to the flow sensor 610 of FIG.6, is attached to the outwardly flared rim 820 surrounding the inletport of the valve, being interposed between the inlet port 858 and theinlet passage 19 in the valve base 6. Staking over of the upper edge 873of the flow sensor housing 864 at several points secures the flow sensor810 to the valve. O-ring seals 21, 860 and 821 prevent leakage past theflow sensor housing 864, between the contacting surfaces of the flowsensor housing 864 and the rim 820, and from the bore 817 to theoutside. A fixed orifice 895 is provided in the flowsensor head 862 forthe purpose discussed in connection with the valve 201.

Bolts co-operating with peripheral flanges (not shown) on the outside ofthe block 818 and threaded bores (not shown) in the valve base 2 may beused in the manner shown in FIG. 1 to secure the valve housing 804 tothe valve base 2. The pilot valve block 803 may be similarly secured tothe block 818 by means of bolts (not shown). Leakage from internal fluidpassages continuing across the boundaries between the base 2 and theblock 818, and the block 818 and the pilot valve housing 803 isprevented by the provision of O-ring face seals of the kind shown forinstance at 55. Strainers such as shown at 869 may be used to filter thefluid supplied to the pilot stage 814.

The pilot valve 814 has a valve spool 848 with two double lands 845A and845B which control the relative proportions of the actuating pressuresin the passages A and B, supplied from the main valve bore 812 at thepressure P₁ via an internal fluid 852 passage to a conventional pressurereducing valve 823 and hence at the reduced pressure P_(R) to the pilotvalve 814.

The pressure reducing valve 823 is retained in a partially threadedbore. As will become clear from the description below of the operationof the cartridge valve, the precise value of the pressure P_(R) suppliedby the pressure reducing valve 823 is not critical to the correctfunctioning of the valve, since only the pressure differential in thefluid ducts A and B derived by means of the pilot valve 814 is ofimportance.

The pilot valve spool 848 is slidably retained at each end in a sealingbush (37, 838) inserted into the pilot valve bore 836. The lower sealingbush 838 incorporates a zero-adjustment screw mechanism 841 for thepilot valve and, when the valve body 804 is assembled, protrudes intothe main valve bore 812, its flanged rim 867 serving also as oneabutment surface of a weak main valve spring 57 whose other end lies onan internal flange 866 of the sleeve 809 and slightly biases the sametowards the "closed" position.

The pilot spool 848 is centered by means of a pair of springs 39, 40 atits outer ends, the lower one of which (40) provides a force adjustableby the aforementioned zero setting screw mechanism 841. An internalpressure duct 854 extending from the inlet passage 19 through the valvebase 2, the block 818 and into the pilot valve block 803 transmits thesupply pressure P_(o) to the lower end chamber 850 of the pilot valve814, and the pressure duct 852, a branch of which leads to the reducingvalve 823, conveys the valve inlet pressure P₁ to the upper end chamber847 of the pilot valve 814. Furthermore, a push rod 42 connects thepilot valve spool 835 to the armature (not shown) of the proportionalsolenoid 5.

In operation, assuming for the purposes of the present explanation thatthe main valve is initially closed, hydraulic fluid is supplied to thevalve through the inlet passage 19 and via a small by-pass hole 895 inthe flow sensor 810. As long as there is no fluid flow, the pressures P₀and P₁ transmitted to the lower and upper end chamber 850, 847respectively, of the pilot valve 814 are equal. Under these conditions,the pilot valve spool 835 is in its neutral position in which the doublelands 845A and 845B of the pilot valve 814 are centred within the outletports 856A and 856B to fluid ducts A and B. Each fluid duct, A and B,transmits half the actuating pressure P_(R), supplied by the pressurereducing valve 823, to the respective actuating surface 828 and 829, thefluid being drained at tank pressure through the upper and lowerdraining duct T respectively. In order to open the main valve, thesolenoid 5 is energised, causing the push rod 42 to move the pilot valvespool 835 in a downward direction. As a result, the actuating pressurein duct A is reduced as the pressure drops across each half of thedouble land 845A are no longer equal; and similarly, the pressure inpassage B is increased. The pressure differential thus applied to thesleeve 809 causes it to move upwards, and with the lower end face 826 ofthe sleeve 809 being withdrawn across the outlet ports 811, fluid startsto flow through the main valve.

As soon as fluid begins to flow a pressure differential between P₀ andP₁ is developed across the flow sensor 810; to begin with across theby-pass hole 895 in the flow sensor bobbin 862 and, once the bobbin 862is displaced against the spring 830, across the annular orifice 855formed between the bobbin 862 and the inner wall of the flow sensorhousing 864. This pressure differential is fed back to the pilot valvespool 835 via the internal pressure ducts 854 and 852 and counter-actsthe solenoid force, thus acting to return the pilot valve 835 to itsneutral position. When the flow reaches the selected value, the pilotvalve spool 835 reaches the neutral position, and the feedback pressuredifferential nominally equals the solenoid force. Thus the pressures onthe actuating surfaces 828 and 829 are once again nominally i.e. apartfrom hydro-dynamic pressures, equal and the sleeve 809 is locked intoposition.

If for any reason, such as for instance an increase in the load (notshown) connected to the outlet passage 13, the flow through the mainvalve changes, the pressure differential across the flow sensor 810changes and the pilot valve spool 835 is moved to cause an upward ordownward movement of the sleeve 809 such that the flow is restored toits selected value. Thus, if the flow decreases, the pressuredifferential decreases, and as it no longer counterbalances the solenoidforce, the pilot valve spool 835 moves downwards initiating thepreviously explained sequence for lifting the sleeve 809 until flow isrestored. If, on the other hand, the flow through the main valveincreases beyond the selected value, the pressure differentialincreases, and the pilot valve spool 835 is moved upwardly from itsneutral position. The actuating pressure in duct A will thereforeincrease and at the same time that in fluid duct B will decrease. Theforce imbalance on the actuating surfaces 828, 829 will thus result in adownward movement of the sleeve 809, causing a reduction in the flow.

The sequence following a change in the selected flow by varying theenergising current to the solenoid 5 will be similar. If the valve is tobe shut, i.e. the flow is selected to be zero, the neutral position ofthe pilot spool 19 will be reached only when P₀ =P₁, i.e. when thesleeve 809 has returned to its lowermost position in which the fluidflow is interrupted.

The valve of FIG. 8 can again be readily adapted to act as by-passcontrol valve, for which the following changes in the lay-out will berequired as previously described with reference to FIGS. 3 to 5:

(a) Fluid flow to the load is made to by-pass the valve by providing afurther outlet passage (not shown) branching off the present inletpassage 19.

(b) The flow sensor 810 has to be relocated into the further outletpassage, and

(c) The feedback operation of the pilot valve 814 has to be reversed.This can be readily achieved, for example, either by an alteration ofthe fluid ducts 854 and 852 such that the pressure P₀ is applied to theupper, and P₁ to the lower end chamber, 847, 850, of the pilot valve814, (reversing the direction of action of the feedback pressuredifferential), or by connecting the fluid duct A to the lower, and thefluid passage B to the upper actuating surface (828, 829) of the sleeve809 (reversing the direction of the actuating pressures on the sleeve809).

In both configurations, i.e. in-line and by-pass, of the valve of FIG.8, the hydraulic feedback transmission may be replaced by electricfeedback transmission such as described with reference to FIGS. 2, 4 and5 above.

It should be noted that expressions such as "upwards", "downwards", etc.are used for convenience in the foregoing description in relation tovalve components and movements, and refer to the orientation of these inthe drawing only--in practice a valve may, of course, be mounted at anydesired angle.

Valve 901 of FIG. 9 is essentially the same as valve 801 in respect ofthe combination and interaction of the integers making up the valve, butshows some modifications in the detailed construction to which thefollowing description will be confined.

The main difference between the valve 801 and the valve 901 is the useof a pressure transducer 991, which converts the feed-back pressuredifferential into an equivalent electric feedback signal. Further minorchanges involve the simplification of the pilot valve 914 which followsfrom the elimination of some of the internal fluid ducts made possibleby the conversion of the feed-back pressure differential into anelectric signal.

The pressure differential developed across the flow sensor orifice isapplied via fluid ducts 954 and 951 to the cantilever beam of aconventional cantilever beam pressure transducer assembly 991 mounted onthe pilot valve housing 903. The pressure differential is applied to thebeam by means of two diameter-matched pins. Alternatively, the pressuredifferential may be applied to a single pin and directly to the beam.The drawing illustrates the two-pin version in which the cantileverbeam, which is shown head-on, bends under the action of the pressureapplied to the outer ends of the two pins and converts the appliedpressure differential into an equivalent electric feed-back signal to beused to control the solenoid 5. This could be done by comparing a demandsignal with the feed-back signal and applying the resultant error signalto control the solenoid.

Employing an electric signal to control the solenoid 5 enables asimplification of the pilot valve spool 935 to a two-double-land spool,and thus contributes towards reducing friction in the pilot valve 914compared to that in the pilot valve 814 of FIG. 8.

We claim:
 1. A hydraulic control valve comprisinga main valve having amain valve inlet port and a main valve outlet port and incorporating afluid pressure actuated valve piston located in a matching valve cavityand having first and second ends, said valve piston cooperating withsaid main valve outlet port for forming a variable metering orifice tocontrol flow from said inlet port to said outlet port, said valve pistonhaving at least one end-to-end fluid duct such that both ends of thevalve piston are exposed to the pressure at the inlet to the meteringorifice, said valve piston ends having equal areas, said valve pistoncomprising a collar defining a first effective control surface area anda second effective control surface area equal to the first effectivecontrol surface area, a pilot stage including a pilot valve and variableflow setting means acting on said pilot valve, fluid control meansthrough which a controlled proportion of fluid pressure acting inoperation of the valve on said second effective control surface area isapplied to said first effective control surface area of the valve pistonthereby to control the variable metering orifice, flow sensing meansbetween the inlet port and the outlet port for generating aflow-dependent feedback signal, and feedback signal transmission meanscoupled to the flow sensing means and to the pilot stage to transmit theflow-dependent feedback signal to the pilot stage to control thevariable flow setting means acting on said pilot valve.
 2. A controlvalve according to claim 1 in which the flow setting means is aproportional solenoid.
 3. A valve as claimed in claim 1 in which thepilot stage comprises a spool valve having a valve spool with two endlands and two intermediate lands, the intermediate lands cooperatingwith control ports in the pilot stage to control the actuating pressuresapplied to respective ones of said control surface areas, the feedbackpressure differential developed across said flow sensing means beingtransmitted to respective ones of the end lands of said pilot stage tocontrol said variable flow setting means.
 4. A valve as claimed in claim1 wherein said pilot stage includes an electromagnetic flow settingmeans, said main stage incorporating fluid ducts transmitting a pressuredifferential developed in operation across the flow sensing means topressure transducer means, the pressure transducer means converting thefeedback differential into an equivalent electric control signal to beapplied to control the electromagnetic flow setting means.